This invention relates to tunnel excavators, and more specifically to a tunnel excavator of the type having a substantially tubular shield, and a rotary cutting disk on one end of the shield carrying a plurality or multiplicity of cutting tools such as cutter rollers. The tunnel excavator of this general configuration is suitable for tunneling in relatively hard soils or rocks.
Mechanical tunneling with shielded excavators is now a standard practice in the earthmoving industry. The shielded tunnel excavator in general cuts the ground by revolving the cutting disk fitted with roller cutters or teeth cutters and advances by the extension and contraction of hydraulic push jacks connected between discrete sections of the shield. Additionally, the excavator has been constructed for supporting the already excavator bore by its skin plate and tail shield and is further provided with a segment errector within the tail shield for lining the tunnel surface with concrete segments or equivalents. The reactive forces that are produced by excavation, by excavator advancement, and by cutting disk rotation have all been transmitted via the push jacks to the erected liner segments thereby to be borne.
Usually, the cutting disk is fitted with teeth cutters (shown at 3 in FIG. 14) for excavating softer soils but with roller cutters (1) for cutting into harder rocks or other formations. The muck or spoil produced by excavation may be water-slurried and discharged through a conduit system, although belt conveyors or screw conveyors are possible and familiar alternative means of spoil disposal. Several problems have been encountered with this type of tunnel excavator.
First, as the tunnel diameter must be greater than the shield diameter for smooth advancement of the excavator, the resulting clearance gives rise to the chance of the rolling of the machine due to reaction from the tunnel face as the cutting disk rotates in cutting engagement therewith. It has been practiced to bidirectionally rotate the cutting disk in order to minimize the rolling. This known practice is objectionable, however, since it may ruin that teeth cutter (3a) opposite to the one acting as scraper (FIG. 14).
Second, excavation must be suspended during liner segment erection after each unit distance advance of the machine, because the reaction of excavation must be borne by the erected segments.
Japanese Pat. No. 920,972 (Publication No. 52-2218) proposes a solution to the above noted problems. The tunnel excavator according to this patent has a shield which is discretely divided into a head section and a tail section, with the latter telescopically nested in the former. This proposed shield construction has its own weaknesses, as discussed in detail hereafter.
The two nested shield sections must have a sufficient clearance therebetween to allow for the steering of the excavator. As the machine is actually steered, the shield sections may go out of alignment with each other to such an extent that a large gap may be created on one side of the machine (FIG. 15). Should the spoil be caught in this gap, the shield sections may become incapable of aligning themselves subsequently. The lapping portions of the shield sections are sealable against spoil intrusion only by use of very complex and expensive means. For this reason the prior art machine permits spoil disposal only by means of a belt conveyor, and not be slurrying the spoil and pumping out the slurry.
A further objection to the above patended machine resides in a pair of fluid actuated cylinders pivotally mounted between the head and tail shields for transmitting the contrarotation of the cutting head to the anchored tail shield during excavation. These cylinders must pivot during the advancement of the machine, as such advancement necessitates the relative axial displacement of the shield sections between which the cylinders are connected. The space requirements of the cylinders are therefore very large, no other equipment being mountable in the paths of their pivotal motions. Still further, the cylinders demand a complex hydraulic circuit as they must be hydraulically interconnected for synchronized operation. It is an additional disadvantage that the cylinders must be of bulky and sturdy construction as they are required to bear the reactive forces of excavation in coaction with the anchors on the rear shield.